This invention relates to a method for removing noise from data. In one aspect this invention relates to a method for removing noise from seismic data where sinusoid type traces are present. The present invention is applicable to the removal of noise from any suitable data. In general, suitable data will be linear, logarithmic or sinusoidal in nature. Examples of such data are seismic data, sonic log data, density log data and other similar geophysical data. However, for the sake of illustration, the invention is described hereinafter in terms of seismic data.
The seismic method of mapping geological subsurfaces of the earth involves the use of a source of seismic energy and reception of the seismic energy by an array of seismic detectors, generally referred to as geophones. When used on land, the source of seismic energy generally is a high explosive charge electrically detonated in a borehole located at a selected grid point in a terrain or is an energy source capable of delivering a series of impacts to the earth's surface such as that used in Vibroseis systems. The acoustic waves generated in the earth by the explosion or impacts are reflected back from pronounced strata boundaries and reach the surface of the earth with varying amplitudes after varying intervals of time, depending on the distance and nature of the subsurface traversed. These returning acoustic waves in the form of sinusoidal type wave forms are detected by the geophones, which function to transduce such acoustic waves into representative electrical signals (generally referred to as "seismic wiggle traces"). The plurality of geophones are arrayed in a selected manner to detect most effectively the returning acoustic waves and generate electrical signals representative thereof. Information may be deduced concerning the geological subsurface of the earth from these electrical signals.
In addition to containing information of interest, seismic wiggle traces may also contain noise. Noise can be described as a high amplitude signal or a high amplitude, high frequency signal imposed on a wiggle trace. This noise may be caused by a plurality of phenomena such as powerline surges, environmental changes or general operation of the seismic exploration system. Also, high amplitude noise referred to as ground roll, tube waves or water bottom multiples may be present in the seismic data. The presence of the noise, especially the high amplitude noise, is a plague to interpreters since the noise may interfere with or mask the desired data which makes computer processing and interpretation of the seismic wiggle traces difficult.
Many attempts have been made in the past to develop methods of removing noise from seismic wiggle traces. The methods must be adaptable to computer processing since the shear volume of the data requires such processing. Also, the method should preferably be simplistic in operation. A brief discussion of some of the methods previously developed follows:
One method is generally referred to as a median filter. The median filter will reorder anomalous amplitudes (noise) to the ends of each data sorted window and will not return a noise value when the median data point of each window is chosen. The disadvantage occurs when data windows do not contain noise which is most of the time. The median value of a noise free window is not necessarily the original data value. Amplitude analysis schemes have also been developed. Such schemes vary but usually work on the principle of comparing the amplitude of each sample to that of a previous sample or to the average of adjacent samples. If the test sample amplitude differs more than a preset factor from the comparator sample, the test sample is revalued to equal the previous sample or the average of the adjacent window samples. This method works well for high amplitude noise but can fail if the noise has low to moderate amplitude.
Derivative schemes have also been developed. Typically, the slope or derivative of a sample is determined and tested against a preset maximum value. The results are good on single sample noise spikes but can fail if a noise burst is amplitude clipped and the slope becomes zero for extended samples.